(厦门大学信息科学与技术学院,福建 厦门 361005)
(School of Information Science and Technology,Xiamen University,Xiamen 361005,China)
DOI: 10.6043/j.issn.0438-0479.201701017
备注
新型二维材料如石墨烯、拓扑绝缘体、过渡金属硫化物和黑磷等不仅具有良好的可饱和吸收特性,而且拥有宽带运转、制备简单、成本低廉和兼容性好等优势,是光调制器件的完美选择之一,被广泛应用于脉冲光纤激光器领域,成为激光研究领域的前沿热点之一.以厦门大学光电子技术研究所近几年的相关研究成果为主要对象,结合国内外研究进展,介绍了二维材料的制备/表征、非线性可饱和吸收特性的测量,并重点阐述了基于二维材料的可见光、近红外至中红外宽波段的调Q/锁模光纤激光器的研究动态.
The novel two dimensional(2D)materials including graphene,topological insulators(TIs),transition metal dichalcogenides(TMDs)and black phosphorous(BPs)not only offer distinct saturable absorption properties,but also possess advantages of broadband operation,low cost and fiber compatibilities.These unique optical properties of 2D materials enable them to be one of the ideal choices for developing amplitude modulators and finding enormous applications in pulsed fiber lasers.In particular,this review focuses on,but not limited to,the research works conducted by the Opto-electronics Technology Institute of Xiamen University in recent years.Fabrication,characterization and saturable absorption measurements of the 2D materials are introduced firstly.Then the overview and progress on visible to mid-infrared broadband fiber lasers,passively Q-switched/mode-locked by 2D materials saturable absorbers,are specially presented.
引言
光纤激光器因其具有结构紧凑、转换效率高、稳定性好等优势在材料加工、光通信、医疗和相关科学的基础研究领域有着非常广阔的应用前景[1].其中脉冲光纤激光器不仅具有光纤激光器的固有优势,同时兼具低功耗、高峰值功率、窄脉宽等特点,在整个激光产业中占据了重要地位,被视为最有潜力的激光光源之一[1-3].
目前获取脉冲激光的最主要途径是调Q和锁模,它们各自又有主动和被动两种实现方式,主动调Q/锁模往往需要在腔内加入一个声光/电光调制器,该方法所获得的脉冲激光虽然可调谐性好,稳定度高,但其成本高昂,特殊波段调制器缺乏,激光器紧凑性难以得到保证[4]; 而被动方式采用可饱和吸收体(SA)作为腔内幅度自调制器件,能够有效克服主动方式的缺点,因此成为当前激光领域的研究热点之一.传统的SA包括金属掺杂晶体[5]、半导体可饱和吸收镜(SESAM)[3]和碳纳米管(CNT)[6-8]等,前两者造价昂贵,难以与光纤兼容,CNT制作较为简单,也易于光学集成,但只能在相对较窄的波段内运行.直到2004年,曼彻斯特大学Novoselov等[9]利用机械剥离石墨的方法成功制造了少层及单层石墨烯,仅仅6年后,他们就获得了诺贝尔物理学奖.此后,以石墨烯为代表的二维材料掀起了科研工作者的研究热潮.
二维材料在作为SA时,不仅具有较好的可饱和吸收特性,还拥有快恢复时间、高损伤阈值、便于制备和光学集成等优点. 2009年,南洋理工大学Bao等[10]和剑桥大学Sun等[11]几乎同时将石墨烯用于锁模超快光纤激光器; 2010年,石墨烯调Q光纤激光器也首次被本课题组Luo等[12]报道.时至今日,基于新型二维材料的脉冲光纤激光器已取得了飞速发展,所涉及的材料包括石墨烯[10,12-18]、拓扑绝缘体(TI)[19-28]、过渡金属硫族化合物(TMD)[29-34]和黑磷(BP)[35-36]等; 光纤集成方式主要有光纤端面光学沉积[12]、二维材料-高分子材料复合薄膜[11,37]、熔锥光纤光学沉积[18,27,38]、侧抛D形光纤光学沉积[23]和光子晶体光纤灌注[39]等; 激光运转波长覆盖了从可见光[36,40-41]、近红外[10-18,22,25-28,30-34,37,42-46]至中红外[14,47-50]光谱波段; 重复频率从几kHz至10 GHz[12,32,34,38,46,51]; 直接输出功率最高达到5 W[52]; 最窄激光脉宽为37.4 fs[53]; 通过啁啾脉冲放大(CPA),全光纤结构的W级fs激光源也已被报道[54]; 这一系列的研究成果表明,二维材料可用于全光谱波段的高性能调Q/锁模光纤激光器的实现,将有望占据未来SA的核心地位.
本综述将从厦门大学光电子技术研究所近几年的工作成果出发,回顾新型二维材料(石墨烯、TI、TMD和BP)的基本性质和非线性可饱和吸收特性在光纤激光器中的应用.
1 二维材料的制备/表征及非线性可饱和吸收特性的测量
1.1 二维材料的制备/表征众所周知,单层石墨烯最早是Novoselov采用机械剥离法制备而得[9],该方法通过使用胶带不断撕离石墨,直至获得少层或者单层厚度的高质量石墨烯,但是该方法效率较低,只能满足实验室小范围需求.一种高可控、可大面积制备二维材料的方法是化学气相沉积法(CVD).以石墨烯为例,利用甲烷等碳化合物和氢气在高温条件下化学反应,通过控制气流速度、反应温度和反应时间等参数就可在Cu或SiC等衬底上生成少层或单层石墨烯薄膜,然后通过化学刻蚀的方式将石墨烯转移至硅片或光纤端面上使用.如图1(a)所示为CVD生长石墨烯的拉曼谱[48],I2D/IG>2,并且表征材料缺陷的D峰非常小,说明CVD制备得到的为高质量单层石墨烯[55].
虽然CVD法可用于批量化生产二维材料,但其工艺一般较为复杂,另一种操作简单、成本低廉并且能够大规模制备二维材料的方法是液相剥离(LPE)法.以MoS2为例,首先将块状MoS2加入到二甲基甲酰胺(dimethyl formamide,DMF)等有机溶液中,超声20 h后,溶液变得均匀,然后通过离心移除剩余的块状和多层MoS2,调节离心速率和时间,可以粗略控制选取的少层MoS2的尺寸和厚度.图1(b)和(c)所示为LPE法制备的少层MoS2的原子力显微镜(AFM)图及其高度图[37],由于单层MoS2厚度约为0.65 nm[56],可知获得的少层MoS2厚度约为3层.
以上两种方法是目前应用最多的方法,且本课题组也多采用此两种方法制备所需的二维材料,而对于其他的制备方法,如分子束外延法(MBE)和水热法等也可用来获取少层或单层二维材料,由于其他方法本课题组在实验中并未涉及,因此这里不展开详述.
1.2 二维材料的非线性可饱和吸收原理由于具备优异的非线性可饱和吸收性能,二维材料已被广泛应用于脉冲光纤激光器中,其具体表现为:当入射光功率足够强时,二维材料对光的吸收不再表现为线性关系,而是随着光强的增加,吸收减小,透过率增加,即可饱和吸收特性.对于二维材料的可饱和吸收特性,通用的模型如图2所示.
如图2(a)所示,当入射光为低强度时,价带电子
图2 二维材料的可饱和吸收原理[10]
Fig.2 Schematic illustration of optical saturable absorption in 2D materials[10]吸收光子能量跃迁至导带,然后这些光生热载流子(电子和空穴)迅速冷却,并最终伴随着声子散射和电子空穴对的复合进入平衡态.由于电子是费米子(自旋为半整数),其遵守泡利不相容原理,也称泡利阻塞(Pauli blocking)——两个全同费米子不能占有同样的量子态.如图2(b)所示,热载流子冷却后遵守费米-狄拉克分布,电子将从最低能量态开始占据一个能量态,价带内的电子重新分布到低能态,空穴则占据高能态.这些受激产生的电子-空穴对在费米能级附近KBTe(KB为玻尔兹曼常数,Te为电子温度)范围内阻止带间跃迁,并相应减弱二维材料对光子的吸收.紧接着电子空穴对重新复合占据主导直到电子和空穴分布恢复到平衡态.当光强足够强时,如图2(c)所示,光生热载流子密度远远大于室温下二维材料的本征电子空穴对密度,最终导带和价带的边缘态不断被填充,当导带和价带的边缘态被完全占据时,从而使得二维材料达到饱和状态,带间跃迁被完全阻断,光子将无损耗通过.
值得注意的是,对于不同的二维材料,其可饱和吸收机理类似,不同的是,石墨烯是零带隙材料,而TI、TMD和BP等则具有一定的能带带隙.
从上面的机理分析可以看出,二维材料的可饱和吸收特性可以用一个二能级系统来描述,其具体表达式如下[57]:
α=(Δα)/(1+I/IS)+αlinear,(1)
T=1-(ΔT)/(1+I/IS).(2)
式(1)和(2)分别为二维材料吸收和透射曲线.其中:α和T是SA的吸收系数和透过率; IS和αlinear为饱和光强和非饱和损耗; Δα和ΔT则分别代表对应的调制深度.
1.3 二维材料的非线性可饱和吸收特性的测量目前测量和表征二维材料的可饱和吸收特性的方法主要有两种:1)Z-扫描系统; 2)平衡双探头测试系统.对于Z-扫描方法,其基本思想是通过测量激光经过光轴上精密移动着的样品之后的功率变化实现对样品材料可饱和吸收特性和非线性系数的测试.该方法直观明了,但对整个系统要求较高,因此本课题组实验中搭建了易于实现的全光纤结构平衡双探头测试系统,如图3(a)所示.整个测试系统由低功率fs级激光种子源、掺铒光纤放大器(EDFA)、可调谐光衰减器(VOA)、3 dB光纤耦合器(OC)、光纤兼容SA及两个功率计组成.其中fs激光种子源为自制的锁模掺铒光纤激光器(EDFL),激射中心波长为1 566 nm,重复频率为22.15 MHz,放大后激光源脉宽为212.1 fs,最大光功率密度为5 GW/cm2,通过设置VOA可以调节二维材料薄片上的入射光功率,同步测量并比较3 dB OC后两臂的输出功率,即可获得二维材料样品的吸收/透射曲线.
如图3(b)~(d)所示分别为采用平衡双探头测试系统测得的单层CVD石墨烯和LPE法制备的少层PVA-MoS2的可饱和吸收曲线,以及PVA-MoSe2的非线性吸收曲线.其中CVD石墨烯采用常用的湿法转移方式转移至光纤端面,而LPE法制备的二维材料则是先与PVA混合,然后在烤箱中烘干成片,最后剪取小片薄片覆盖于光纤端面,构成“三明治”型的光纤兼容SA.基于式(1)和(2)拟合得到3种二维材料的调制深度和饱和光强分别为0.86%、1.60%、0.63%和0.8,13.0,19.8 MW/cm2,较低的饱和光强将有助于低阈值调Q/锁模的建立.值得一提的是,如图3(d)所示,在入射光功率较高时(>260 MW/cm2),观察到少层MoSe2存在反饱和吸收,发生该现象的原因并非是样品的热损伤,而是材料的双光子吸收(TPA)所致,采用公式[58]:
α=βI2+αlinear,(3)
拟合得到MoSe2的TPA系数β为3.4×10-6 cm4MW2.
2 二维材料被动调Q光纤激光器
众所周知,调Q是获取大脉冲能量(10-9~10-6 J)、大脉冲宽度(10-7~10-6 s)及低重复频率(kHz)脉冲激光的最主要方法,早在20世纪60年代,科学家们就已经进行了这方面的尝试[59].到2000年以前,受限于性能优良、制备简单和集成便利的SA的缺乏,被动调Q光纤激光器的发展非常缓慢.21世纪初以来,CNT和石墨烯等低维纳米材料可饱和吸收特性[7,10-11,19,29,35]的发现,给被动调Q光纤激光器的研究带来了新的契机.本课题组瞄准这一主题,围绕着1)充分开发新型二维材料的超宽带可饱和吸收特性,2)提升调Q单脉冲能量,3)紧凑结构激光器,开展了一系列实验研究,获得了高性能的可见光、近红外至中红外的被动调Q激光.
2.1 可见光被动调Q短脉冲光纤激光器TMD具有对应于可见光波段的带隙,因而具有强的可见光吸收特性; TI具有无能隙的表面金属态,其吸收带从可见光延伸到中红外; BP是一种带隙与原子层数相关的高迁移率半导体材料,其单层能隙对应于可见光波段,也具有强的可见光吸收; 纳米金属材料由于表面等离子体共振效应同样对可见光具有良好的吸收和散射特性[60].虽然这些低维纳米材料具有优良的可见光吸收特性,且其在可见光波段的可饱和吸收性能早已被表征[61-64],但其在可见光激光器的应用一直鲜有报道.
针对这一现状,本课题开展了低维纳米材料红光波段光纤激光器的调Q实验研究.首先分别将LPE法制备的TMD(WS2、MoS2和MoSe2)、TI(Bi2Se3和Bi2Te3)、BP以及纳米金属材料(纳米金(GNP)和纳米铜(CuNW))与PVA混合,烘干成膜制备成与光纤兼容SA,并插入同一个红光光纤激光器中,获得了稳定的调Q运转,并进一步深入对比与分析了这些低维纳米材料在红光波段的调Q性能.激光器的原理图如图4所示,最大输出功率2 W的445 nm氮化镓(GaN)蓝光激光二极管(LD)通过准直聚焦系统耦合入一段98.5 cm的掺镨氟化物(Pr:ZBLAN)光纤(纤芯/包层:6/125 μm,Pr3+掺杂量为0.1%(摩尔分数))中,掺镨光纤4%端面菲涅尔反射和镀上红光高反膜的光纤连接头构成激光器的两个反射镜,激射的红光背向从4%端输出,并通过斜放在准直聚焦系统中的红光高反镜耦合出激光腔体,激光器有效腔长为1.97 m.
本课题组对比了各种低维纳米材料红光被动调Q光纤激光器的输出特性(表1)[36,40-41,65-66].从表中可知,激光器激射波长都在635 nm附近,调Q脉冲信噪比RSN都在40 dB以上,表明这些低维纳米材料都可作为有效的红光Q开关.但是,其调Q输出性能存在一定的差别:1)对于调Q脉冲,TMD、Bi2Se3和GNP的脉宽更窄,而Bi2Te3、BP和CuNW的脉宽更宽.一般地,被动调Q脉宽受腔长和SA调制深度的影响,由于二维材料厚度极小,其对腔长的影响可以忽略,因此影响脉宽的主要因素是材料的调制深度,可以得出在实验过程中TMD、Bi2Se3和GNP的调制深度较Bi2Te3、BP和CuNW理想,以WS2为最佳.2)对于调Q脉冲的单脉冲能量和激光器的输出功率,TMD大体较为理想,特别是MoS2和MoSe2,平均输出功率可达20 mW左右,单脉冲能量可在50 nJ左右.因此,在实验过程中,TMD较其他材料更能有效地调制红光,使激光器输出高性能红光调Q脉冲.
2.2 1,1.5和2 μm被动调Q短脉冲激光器除可见光波段外,本课题组也开展了二维材料被动调Q 1 μm掺Yb3+光纤激光器(YDFL)、1.5 μm 掺Er3+(EDFL)和2 μm 掺Tm3+(TDFL)光纤激光器的实验研究.图5(a)~(c)为1 μm调Q YDFL的实验装置原理图、光谱图和典型脉冲序列图[22],实验中采用两个高反射率光纤布拉格光栅(FBG)构建紧凑的线性腔,一段20 cm高掺杂YDF(Nufern,SM-YSF-HI,250.0 dB/m@975 nm)提供增益,少层TI:Bi2Se3通过光学诱导沉积至光纤端面组装成调Q器件,测试光谱显示调Q激光具有两个波长1 067.66 nm和1 067.91 nm,这是FBG2的两个反射峰所致,调Q最窄脉宽为1.95 μs,单脉冲能量为17.9 nJ,将FBG2换成一个输出耦合比约为15%的FLM,并使用50 cm的高掺杂YDF作为增益光纤,单脉冲能量可提升至41.3 nJ,这是首次将TI的可饱和吸收特性向短波长推移至1 μm波段,此外,本课题组也进一步将TI用于调Q 1.5 μm EDFL[67]和大能量 2 μm TDFL[21].
值得说明的是,TI具有一定的能带带隙,如Bi2Se3约为0.3 eV,对应波长约为4.1 μm,但其表面存在非常稳定、不受杂质及外部环境影响的无能带间隙的金属态[68],当入射波长>4.1 μm时,表面金属态起到可饱和吸收的作用,而当入射波长<4.1 μm时,其体内和表面都可贡献于非线性可饱和吸收,具有大的归一化调制深度(可达98%[45]),因此TI可用于制备全光谱波段的调制器件.
与石墨烯的零带隙和TI的表面金属态类似,少层TMD(如MoS2)的边缘态可以调整其过渡金属原子与硫族原子的比例,从而有效调控能带带隙,使其具备宽带可饱和吸收特性[69-70].基于此,本课题组首先通过采用单一MoS2-SA(可饱和吸收特性见图3(c))分别实现了1 μm YDFL、1.5 μm EDFL和2 μm TDFL被动调Q光纤激光器[71],实验验证了MoS2的宽带可饱和吸收特性.其中2 μm调Q TDFL装置与特性如图5(d)~(f)所示.少层MoS2通过LPE法制备并与PVA混合烘干成薄膜,然后嵌入光纤端面构成“三明治”型SA,增益光纤为5 m 双包层TDF(Nufern,SM-TDF-10P/130-HE); 调Q激光激射波长为2 032 nm(图5(e)); 图5(f)为典型的调Q脉冲序列,脉冲时间间隔为26 μs,调Q最窄脉宽为1.76 μs,最大单脉冲能量约为1 μJ,如此大的脉冲能量得益于双包层TDF的强劲增益.该实验佐证了MoS2不仅具有宽带可饱和吸收特性,并且也能够支持大脉冲能量的产生,因此可将MoS2-SA进一步用于波长可调谐调Q光纤激光器的实现.如图5(g)所示为可调谐调Q EDFL的实验装置图,采用一个自由光谱范围(FSR)为48.5 nm、中心波长为1 539 nm 的F-P可调谐滤波器(Micron Optics Inc.)调谐腔内激射波长.固定泵浦功率为46.1 mW,调节加载在滤波器上的电压,EDFL能在宽达48.1 nm(1 519.6~1 567.7 nm,从S-L波段)范围内实现调Q输出(图5(h)).这一数值与石墨烯[16](30 nm)调谐EDFL相比拟,远好于SWNT[6](5 nm).实际上,本实验的调谐范围受限于F-P滤波器的FSR(48.5 nm),而非MoS2-SA本身,如果使用FSR更大的滤波器,调谐范围将能覆盖整个Er3+增益波段.固定激射波长为1 551.2 nm,调Q重复频率在8.77~43.47 kHz内可调,最小脉宽为3.3 μs,脉冲能量达到160 nJ,调Q性能在普通单模EDFL中已经相当可观.
表2 典型低维纳米材料被动调Q 1,1.5及2 μm光纤激光器的性能比较
Tab.2 Comparison of the typical 1,1.5 and 2 μm passively Q-switched fiber lasers with different low-dimensional nano-materials as saturable absorbersLPE法制备的二维纳米材料往往呈现为小块碎片,制作成薄片时需要加入PVA作为载体,在激光运转中容易引入散射损耗,产生的热效应对SA性能有着较为不利的影响,而CVD法制备的二维材料可以改善这一劣势,本课题组通过将单层CVD石墨烯(可饱和吸收特性见图3(b))转移至光纤端面,实现了脉冲能量1.05 μJ、调谐范围约为16 nm的1.5 μm EDFL可调谐调Q[72],1 μm和1.5 μm双色同步被动调Q[73],其中1 μm脉冲能量高达5.3 μJ,以及脉冲能量为1.06 μJ的 2 μm TDFL被动调Q激光[48].
表2给出了国内外及本课题组[74-79]低维纳米材料被动调Q光纤激光器的性能比较,包括输出功率、脉冲能量、最小脉宽和重复频率调谐范围等参数,可以看到,本课题组在该领域取得了较为突出的成果,如首次利用TI:Bi2Se3实现1 μm YDFL的调Q[22]及首次实验验证了少层MoS2的宽带可饱和吸收特性等[71],并将调Q单脉冲能量提升到μJ量级[48,72].
3 二维材料被动锁模皮秒/飞秒光纤激光器
锁模激光器拥有超窄脉宽(fs~ps)、超高峰值功率(>数十W)和高相干性等特点[2].被动锁模超快光纤激光器由于无需在激光腔内加入主动调制器件,拥有成本低、结构紧凑、设计自由度高及噪声低等优势而深受青睐.自1990年以来[80],锁模光纤激光器得到了迅速发展.如今,已有40多家公司经营销售超快光纤激光器,其应用遍布材料加工、医疗、精密测量和光通信等领域[2].一方面,被动锁模以幅度调制的SA作为启动开关,对于性能更优成本更低的新型SA有着持续的挖掘动力.另一方面,由于光纤激光器腔长较长,脉冲在腔内循环,增益、增益色散、光纤色散和损耗等的综合作用使得光纤激光器的锁模现象尤其丰富,探究被动锁模光纤激光器的内在规律、开发新型功能超快激光也是重要的研究课题.本课题组以研究新型二维材料SA、探索锁模新现象和拓宽锁模光谱波段为出发点,针对二维材料锁模超快光纤激光展开了诸多工作.
图6 石墨烯DS锁模1 μm YDFL
Fig.6 Dissipative soliton generation in a passively mode-locked 1 μm YDFL based on graphene3.1 二维材料锁模1 μm皮秒激光器全正色散锁模光纤激光器中形成的耗散孤子(DS)是腔内非线性、色散、增益和损耗等效应共同作用的结果[81].使光纤的非线性效应和色散导致的脉冲啁啾累积以及增益与损耗所导致的幅度调制保持平衡,在更大的非线性相移下而保证光孤子不分裂,这是产生高能超快脉冲的主要方法之一.
利用石墨烯,本课题组实现了三波长运转的1 μm YDFL的DS锁模.实验中所采用的石墨烯通过化学还原氧化石墨烯(GO)获得,厚度约为3~4层,再进一步嫁接至聚丙烯酸上获得了高水溶性的石墨烯,最后通过光学诱导法,将石墨烯有效地沉积至熔锥光纤(锥腰直径约为3.5 μm)上.通常,熔锥光纤会引入一定的周期性滤波效应[82],当它插入谐振腔时,将与腔内的双折射效应共同构成一个有效的Lyot滤波器[83].并且由于石墨烯较强的偏振效应[84],能够明显增强熔锥光纤器件的滤波特性(图6(a)内嵌图).因此,在该实验中,沉积有石墨烯的熔锥光纤扮演了双重角色:1)作为SA,启动DS锁模; 2)作为周期性滤波器,实现谐振腔多波长激射.YDFL的装置原理图如图6(a)所示,25 cm高掺杂YDF(双折射系数为2.5×10-4)提供增益,368 m掺磷光纤可以在增加腔内非线性的同时,降低锁模重复频率.
YDFL大多数情况下处于三波长非孤子锁模状态,调节偏振控制器(PC),激光可以从非孤子锁模态转变为DS锁模状态,其锁模光谱具有陡峭的边沿,3个激射波长的中心波长为1 031.43,1 034.94和1 038.43 nm(如图6(b)).图6(c)记录了1 038.43 nm DS锁模的射频(RF)频谱特性,锁模重复频率约为551.5 kHz,基频频谱的RSN为62.5 dB,显示DS锁模的稳定运转; 内嵌图则给出了10 MHz宽带范围内的频谱,频谱上没有发现大的幅度调制和噪声分量,说明此时腔内处于良好的连续波锁模状态.图6(d)所示为测量的DS锁模自相关迹,采用Gaussian函数拟合,得到脉宽为74.6 ps,对应时间脉宽积(TBP)为31.5,表明锁模脉冲存在大的啁啾.采用一个弱啁啾的FBG(带宽为1.9 nm,群速度色散(GVD)约为10 ps2/nm),可将输出脉冲宽度压缩至65.2 ps,如果采用啁啾更大的FBG或者光栅对,DS脉冲将有望被压缩至更窄宽度.DS锁模单脉冲能量为6.4 nJ,这主要得益于实验中将石墨烯沉积至熔锥光纤上,有效增加了光与石墨烯的作用距离,改善了石墨烯SA的热效应.
图7 GO-MoS2锁模1.5 μm EDFL
Fig.7 Passively mode-locked 1.5 μm EDFL based on GO-MoS23.2 二维材料锁模1.5 μm飞秒激光器1.5 μm EDFL通常工作在反常色散区,该区域的锁模孤子一般通过光纤的非线性效应和腔内色散的平衡作用而达到稳定,啁啾较小,因此容易获得超窄脉宽(fs)的锁模脉冲.实验中采用GO作为胶体表面活性剂替代DMF或N-甲基吡咯烷酮(NMP)等有机溶剂剥离少层MoS2,一方面可以获得厚度较为统一、不易团簇的少层MoS2,另一方面也能改善MoS2的载流子迁移率.经过测试对比,GO-MoS2在引入小得多的线性损耗下,就能获得与DMF-MoS2相比拟的可饱和吸收性能.利用平衡双探头测试系统,测得GO-MoS2-SA初始透过率为75.3%,调制深度为1.51%,饱和光强为92 MW/cm2.EDFL装置原理图如图7(a)所示,整个腔长约为26.2 m,腔内总色散为-0.23 ps2.图7(b)所示为典型锁模光谱,3 dB带宽为1.9 nm,并具有明显的Kelly边带,其中一阶边带距中心波长为4.46 nm,利用自相关仪测得其脉宽为1.9 ps(图7(c)),计算得到TBP为0.446,略大于传输极限0.44,表明锁模脉冲只有很小的啁啾.图7(d)所给出的频谱图也显示了该EDFL具有良好的锁模性能.
另外,通过使用端面镀膜光纤构建紧凑的F-P线性腔,并调控腔内色散,获得了双向输出的10-13 s级的束缚态锁模孤子,其中前向和后向输出锁模脉宽分别为173 fs和182 fs[85].
3.3 二维材料锁模2 μm激光器锁模TDFL产生的2 μm超快激光具有超短脉冲宽度和较高的峰值功率,在激光手术、气体传感和中红外光谱学中有着重要应用.因此,对于超快锁模TDFL的实现不仅具有重要的科研价值,也是直接而迫切的现实需要.
为了构建2 μm TDFL,本课题组在实验中首先搭建了1 565 nm 双包层EDFL作为泵浦源,输出功率>1 W.如图8(a)所示,在TDFL中,一段1.2 m Tm3+-Ho3+共掺光纤(THDF,吸收系数约为16 dB@1 565 nm)作为增益光纤,整个腔长约为13.26 m,其中THDF的GVD为-73 ps2/km,腔内其他光纤均为SMF-28e,GVD为-71 ps2/km,因此整个谐振腔处于全负色散区,总色散约为-0.94 ps2.锁模器件同样为GO-MoS2-SA,锁模中心波长为1 910.5 nm(图8(b)),激射波长较短的原因在于腔内2 μm 波段器件如ISO、OC和PC等都具有相对较大的损耗,由于光谱仪较低的分辨率(6.3 nm),导致实验中无法观察到Kelly边带的存在和精确测量光谱3 dB带宽.图8(c)为典型的锁模脉冲序列,脉冲周期为64.4 ns.图8(d)显示锁模基频频谱RSN>62 dB,说明锁模稳定性良好.实验中继续增加泵浦功率,激光器发生谐波锁模,并表现出能量量子化效应,最高阶次为12阶,重复频率达到187.18 MHz,继续优化GO-MoS2-SA和腔结构设计,将有望获得高重频的2 μm锁模激光.遗憾的是,由于自相关仪配件的缺乏,锁模激光的脉冲宽度无法准确知晓.
图8 GO-MoS2锁模2- μm TDFL
Fig.8 Passively mode-locked 2- μm TDFL based on GO-MoS2另外,实验中还搭建了1 212 nm拉曼光纤激光器泵浦THDF,与1 565 nm泵浦方案相比,光-光转换效率提升了近1倍,并采用石墨烯作为SA获得了性能稳定的锁模输出[86].
综上,在低维纳米材料被动锁模光纤激光器研究方面,本课题组紧跟国内外同行步伐,也取得了一系列成果,并且针对可饱和吸收特性如何影响被动锁模激光性能的问题,分别开展了相关的理论和实验研究[87].结果表明,在反常色散区锁模脉冲宽度随着SA调制深度的增加而减小,而SA的可饱和光强对锁模性能的影响较小.表3[88-99]根据SA的种类分别罗列了部分低维纳米材料被动锁模光纤激光器的性能.当然,低维纳米材料被动锁模光纤激光器的研究已取得了长足进展,并不仅限于表3中所列,如Ma等[53]使用石墨烯获得了EDFL锁模直接输出的最短脉冲,他们通过缩短腔长,得到的脉宽为37.4 fs; 2013年,Li等[43]在激光腔内插入GO薄片,实现了耗散孤子锁模,获得了迄今为止二维材料锁模光纤激光器输出最大单脉冲能量429 nJ.二维材料的发现也极大地促进了2 μm锁模激光的发展,使用机械剥离少层BP作为SA,2 μm fs级(739 fs)锁模脉冲得以实现[99].
4 总结与展望
在本综述中,从课题组过去几年的研究成果出发,首先阐述了几种二维材料的基础性质,包括材料制备/表征方法和非线性可饱和吸收机理,并介绍了目前其可饱和吸收特性的测试方法.然后详细列举了目前二维材料调Q/锁模光纤激光器的研究进展,基于石墨烯、TI、TMD和BP等新型二维材料,实验实现了性能良好的可见光、近红外至中红外波段调Q/锁模光纤激光器,其中本课题组多项工作都处于该领域前沿水平.值得强调的是,二维材料不仅拥有好的可饱和吸收性能,而且宽带运转、制备简单、成本低廉、兼容性好,是光纤型幅度调制器件的理想材料,能够广泛应用于脉冲光纤激光器的制作.未来的发展方向仍应围绕二维材料的宽带工作特性及其在光纤激光器应用中的易集成性,将其拓展到中红外波段和紫外波段的脉冲激光应用中.另外,也可开发二维材料的其他光电学特性,如偏振效应、强的Kerr非线性效应和快速载流子跃迁等,制备光纤型二维材料光电器件.
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